If you have celiac disease, gluten causes your body to attack itself, damaging your intestines. So it’s essential to avoid the stuff—but a new study suggests that restaurants’ “gluten-free” labels are often wrong.

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The study wasn’t able to determine how trustworthy the labels are, so eating out is still a bit of a minefield—which, if you have celiac, you already know.

The data comes from a company, Nima, that sells a gluten detector. It’s expensive, at $289 for the starter kit plus $6 for each one-use test capsule. If you pair the device with your phone, you can choose to “share” your test results with the company. They used those shared results to try to figure out what’s in the foods people are testing.

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The results: a lot of the foods came up as gluten-positive, about a third of them on average, including half of the pizza and pasta samples. Positive results were more likely at dinnertime than during other meals. Does this mean you should avoid restaurant pasta dinners? Maybe.

This study has some serious limitations, though

To be clear, these are foods that people who own a portable gluten sensor decided were suspicious enough to test. (Even a very accurate test will turn up a lot of false positives if it’s getting used a lot on gluten-free foods, so that’s important to keep in mind.) The authors of the study also note that people may have been more likely to share their results if they were positive. Sharing the results requires the user to answer questions about the food, which I’m guessing even the dedicated users don’t do at every meal.

There’s one other caveat. The Nida device is very sensitive—possibly too sensitive. Packaged foods can be considered “gluten-free” if they contain less than 20 ppm of gluten, but Nida doesn’t report an amount of gluten found. Instead, it gives a yes-or-no answer, often flagging foods with 5 or 10 ppm of gluten. Does that make it really good, or nearly useless? It’s hard to say.